Please refer to FIG. 5, which is the cross-sectional schematic showing an embodiment of the ohmic metal for GaN device of conventional technology. The ohmic metal 9 for GaN device of conventional technology comprises a first metal layer 91, a second metal layer 92, a third metal layer 93 and a fourth metal layer 94. The first metal layer 91 is formed on an AlGaN/GaN epitaxial structure layer 90 by physical vapor deposition (PVD), wherein the first metal layer 91 is made of Ti. The second metal layer 92 is formed on the first metal layer 91 by physical vapor deposition, wherein the second metal layer 92 is made of Al. The third metal layer 93 is formed on the second metal layer 92 by physical vapor deposition, wherein the third metal layer 93 is made of Ni, Ti or Mo. The fourth metal layer 94 is formed on the third metal layer 93 by physical vapor deposition, wherein the fourth metal layer 94 is made of Au. The ohmic metal 9 for GaN device of conventional technology (including the first metal layer 91, the second metal layer 92, the third metal layer 93 and the fourth metal layer 94) further needs a rapid thermal annealing (RTP) process treatment, wherein a rapid thermal annealing temperature of the rapid thermal annealing process and a rapid thermal annealing time of the rapid thermal annealing process are related to the material of the ohmic metal 9 and the thickness of the ohmic metal 9. In an embodiment, the ohmic metal 9 has the same structure as the ohmic metal 9 for GaN device of conventional technology in FIG. 5. The first metal layer 91, the second metal layer 92, the third metal layer 93 and the fourth metal layer 94 of the ohmic metal 9 are made of Ti, Al, Ni, Au respectively. The thicknesses of the first metal layer 91, the second metal layer 92, the third metal layer 93 and the fourth metal layer 94 of the ohmic metal 9 are 20 nm, 100 nm, 55 nm and 55 nm respectively. In current embodiment, the rapid thermal annealing temperature of the rapid thermal annealing process is between 880° C.˜925° C. The rapid thermal annealing time of the rapid thermal annealing process is between 20 seconds˜60 seconds. The disadvantage of current embodiment is that the breakdown voltage of the GaN field effect transistor (FET) having the ohmic metal 9 is about 120V. And the values of the breakdown voltage of the GaN field effect transistors are widely distributed. However for a high energy density GaN field effect transistor application, the breakdown voltage is required to be higher. And the values of the breakdown voltage of the GaN field effect transistors cannot be widely distributed to ensure the values of the breakdown voltage fall within the specifications.
In another embodiment, the ohmic metal 9 has the same structure as the ohmic metal 9 for GaN device of conventional technology in FIG. 5. The first metal layer 91, the second metal layer 92, the third metal layer 93 and the fourth metal layer 94 of the ohmic metal 9 are made of Ti, Al, Ti, Au respectively. The thicknesses of the first metal layer 91, the second metal layer 92, the third metal layer 93 and the fourth metal layer 94 of the ohmic metal 9 are 20 nm, 100 nm, 55 nm and 55 nm respectively. In current embodiment, the rapid thermal annealing temperature of the rapid thermal annealing process is between 845° C.˜875° C. The rapid thermal annealing time of the rapid thermal annealing process is between 20 seconds˜60 seconds. Please refer to FIG. 6A, which is the top view of the SEM image of an embodiment of the ohmic metal (Ti/Al/Ti/Au) for GaN device of conventional technology. In FIG. 6A, it is obviously that a top surface of the ohmic metal is non-uniform, rough and not smooth. Please also refer to FIGS. 6B and 6C, which are the partial enlarged views of the SEM image of the embodiment of FIG. 6A. In FIG. 6B, it is clear that the top surface of the ohmic metal not only presents non-uniform, but also presents serious protrusion and depression. In FIG. 6C, the ohmic metal also presents non-uniform and even presents protrusion beyond the edge. Please also refer to FIG. 6D˜6F, which are the cross-sectional views of the FIB (focused ion beam) image of three embodiments of the ohmic metal (Ti/Al/Ti/Au) for GaN device of conventional technology. These three embodiments show the partial cross-sectional views of three GaN field effect transistors. Firstly the three GaN field effect transistors are polished by cross-section polish, and then the three GaN field effect transistors are scanned over by the focused ion beam for generating the image. The S1, S2 and S3 regions of the three embodiments all present serious tumor at the edge of the ohmic metal. The phenomenon is consistent with the phenomenon of protrusion and depression observed in FIG. 6B. Therefore, the disadvantages of the embodiments of conventional technology are that after the rapid thermal annealing process treatment, the top surface of the ohmic metal becomes very non-uniform and generates serious tumor at the edge of the ohmic metal. These drawbacks will affect the characteristics of the GaN devices having the ohmic metal for GaN device of conventional technology and affect the result of the reliability test of the GaN devices having the ohmic metal for GaN device of conventional technology.
In another embodiment, the ohmic metal 9 has the same structure as the ohmic metal 9 for GaN device of conventional technology in FIG. 5. The first metal layer 91, the second metal layer 92, the third metal layer 93 and the fourth metal layer 94 of the ohmic metal 9 are made of Ti, Al, Mo, Au respectively. The thicknesses of the first metal layer 91, the second metal layer 92, the third metal layer 93 and the fourth metal layer 94 of the ohmic metal 9 are 15 nm, 75 nm, 40 nm and 55 nm respectively. In current embodiment, the rapid thermal annealing temperature of the rapid thermal annealing process is 865° C. The rapid thermal annealing time of the rapid thermal annealing process is between 20 seconds˜60 seconds. Please refer to FIG. 7A, which is the top view of the SEM image of an embodiment of the ohmic metal (Ti/Al/Mo/Au) for GaN device of conventional technology. Please also refer to FIG. 7B, which is the cross-sectional view of the TEM image along the broken line of the arrow of the embodiment of FIG. 7A. Firstly the embodiment is polished by cross-section polish, and then the embodiment is scanned over by transmission electron microscopy for generating the image. In FIG. 7B, the ohmic metal (the left side and the right side) is covered by a silicon nitride layer. The ohmic metal (the left side and the right side) presents very non-uniform. Please also refer to FIG. 7C, which is the partial enlarged view of the TEM image of the block R region of the embodiment of FIG. 7B. Obviously, the ohmic metal is very non-flatly distributed. Furthermore, there exists the ohmic metal spiking phenomenon. Please also refer to FIGS. 7D, 7E, 7F and 7G, which are the partial enlarged views of the TEM image of the block R1, R2, R3 and R4 regions of the embodiment of FIG. 7C respectively. The ohmic metal in each of the R1, R2, R3 and R4 regions of the embodiment is very non-uniform. Moreover, the ohmic metal spiking phenomenon extends downward to the AlGaN/GaN epitaxial structure layer 90. Hence, the disadvantages of the embodiment of conventional technology are that, after the rapid thermal annealing process treatment, the ohmic metal is very non-uniform and there exists the ohmic metal spiking phenomenon. These drawbacks will affect the characteristics of the GaN devices having the ohmic metal for GaN device of conventional technology and affect the result of the reliability test of the GaN devices having the ohmic metal for GaN device of conventional technology.
Accordingly, the present invention has developed a new design which may avoid the above mentioned drawbacks, may significantly enhance the performance of the devices and may take into account economic considerations. Therefore, the present invention then has been invented.